1. Field of the Invention
This invention relates generally to signal frequency synthesis and more particularly to a frequency synthesizer and method for synthesizing exact multiples of decimal frequencies.
2. Description of the Background Art
Microwave frequency synthesizers play a crucial role in consumer, industrial and military applications. They are found in test systems for cell phones, radars and communication transceivers. The performance, cost and size of a synthesizer has a very substantial impact on commercial success and operational viability in numerous instances. Low phase noise, low spurious signal levels and fast switching speed are key performance factors.
Architectures for microwave frequency synthesis fit into the two broad categories of direct synthesis and indirect synthesis. The direct synthesis architectures use analog components such as mixers, filters, switches, frequency multipliers and frequency dividers for simple direct arithmetic frequency operations of addition, subtraction, multiplication and division. The resulting solutions have high performance, but when applied in the microwave frequency range, are costly, bulky and have high power consumption.
Indirect synthesis architectures make use of phase lock loops (PLL)s in the signal-switching-path. Single switching loop PLL architectures have low cost, small size and low power consumption by virtue of their simplicity. Such simple architectures are currently used in applications that do not require the level of performance demanded in military and high volume test applications. However, to meet a high level of performance, existing products use multiple phase-locked loops, sometimes as many as 8 or 10, in the signal switching path. As a result, the PLL implementations of high performance frequency synthesizers are also costly and bulky, and have high power consumption.
Both direct and indirect frequency synthesis use a reference frequency derived from a base reference oscillator for providing a highly stable, low phase noise signal from which all other frequencies are synthesized. While it is theoretically possible to use a base reference oscillator at any frequency, an oscillator having a decimal frequency of 10 MHz or 100 MHz is highly preferred because a great deal of engineering has gone into achieving high stability and low phase noise for the lowest cost for oscillators having these frequencies. Relatively low cost binary devices are used for fractional multiplication of the reference frequency for synthesizing the desired output frequency.
Frequency synthesis of exact multiples of 10 kHz, 100 kHz or 1 MHz is often desirable for communication and radar receiver test, and direct use as local oscillators in receivers and transmitters. These receivers and transmitters commonly use channel spacings that have exact decimal Hertz values (sums of multiples of 100 kHz). For example, GSM may have a channel spacing of exactly 200 kHz. Unfortunately, a 10 MHz or 100 MHz reference oscillator with fractional binary multiplication cannot directly provide steps sizes to exactly match these channel spacings. Of course, this problem can be resolved with ultra-fine resolution so that the frequency difference between the desired and actual step size is negligible. However, ultra-fine frequency resolution is expensive to achieve. The problem could be resolved using a binary base reference frequency, for example 225 at 33.554,432 MHz or 226 at 67.108864 MHz. However, base reference oscillators at these frequencies do not have all the benefits of existing base reference oscillators having decimal base frequencies of 10 MHz or 100 MHz.
There is a need for a low phase noise, low spurious signal level, fast switching microwave synthesizer at a low cost having frequency steps at exact multiples of a decimal frequency using a decimal frequency base reference frequency.